The present invention relates to electronic ballasts for gas discharge lamps, such as fluorescent lamps.
Electronic ballasts for fluorescent lamps typically can be analyzed as comprising a xe2x80x9cfront endxe2x80x9d and a xe2x80x9cback endxe2x80x9d. The front end typically includes a rectifier for changing alternating current (AC) line voltage to a direct current (DC) bus voltage and a filter circuit for filtering the DC bus voltage. The filter circuit typically comprises an energy storage capacitor. Electronic ballasts also often use a boost circuit for boosting the magnitude of the DC bus voltage. Additionally, an electronic ballast is known that uses passive power factor correction means to reduce ballast input current total harmonic distortion. These means include line frequency filter circuits having a high impedance at line frequency and about the first 30 harmonics of the line frequency. The high impedance of line frequency filter circuits have a significant reducing effect on ballast input current total harmonic distortion. These filters are in contrast to EMI filters which have low impedance at line frequency and the related harmonics and therefore have no significant effect on ballast input current total harmonic distortion.
The ballast back end typically includes a switching inverter for converting the DC bus voltage to a high-frequency AC voltage, and a resonant tank circuit having a relatively high impedance for coupling the high-frequency AC voltage to the lamp electrodes. The ballast back end also typically includes a feedback circuit that monitors the lamp current and generates control signals to control the switching of the inverter so as to maintain a desired lamp current magnitude.
In order to maintain stable lamp operation, typical prior art electronic ballasts filter the DC bus voltage to minimize the amount of bus voltage ripple. This is usually accomplished by providing a bus capacitor having a relatively large capacitance and hence, a relatively large energy storage capacity. By providing a relatively large bus capacitor, the amount of decay from the rectified peak voltage is minimized from one half-cycle to the next half-cycle. Minimizing the amount of ripple on the DC bus also tends to minimize the current crest factor (CCF) of the lamp current. The CCF of the lamp current is defined as the ratio of the magnitude of the peak lamp current to the magnitude of the root-mean-square (RMS) value of the lamp current.                     CCF        ≡                              I            pk                                I            RMS                                              (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
An important indicator of lamp current quality for a gas discharge lamp such as a fluorescent lamp is the current crest factor (CCF) of the lamp current. A low CCF is preferred because a high CCF can cause the deterioration of the lamp filaments which would subsequently reduce the life of the lamp. A CCF of 2.1 or less is recommended by Japanese Industrial Standard (JIS) JIS C 8117-1992, and a CCF of 1.7 or less is recommended by the International Electrotechnical Commission (IEC) Standard 921-1988-07.
However, using a relatively large bus capacitor to minimize ripple on the DC bus voltage comes with its disadvantages. The larger the bus capacitor, the more expensive it is, and the more area it consumes on a printed circuit board, or the like, and the more volume it uses within the ballast. Also, the bus capacitor is discharging whenever the bus voltage level is above the instantaneous absolute value of the AC line voltage, and hence the bus capacitor recharges only during a relatively short time within each line half-cycle, around the absolute value peak voltage of the AC line voltage. Thus, typical prior art ballasts draw a relatively large amount of current during the short time that the bus capacitor is charging, as shown in FIG. 1. This results in a distorted ballast input current waveform giving rise to unwanted harmonics and undesirable levels of total harmonic distortion (THD).
In an AC power system, the voltage or current wave shapes may be expressed as a fundamental and a series of harmonics. These harmonics have some multiple frequency of the fundamental frequency of the line voltage or current. Specifically, the distortion in the AC wave shape has components which are integer multiples of the fundamental frequency. Of particular concern are the harmonics that are multiples of the 3rd harmonic. These harmonics add numerically in the neutral conductor of a three phase power system. Typically, total harmonic distortion is calculated using the first 30 harmonics of the fundamental frequency. Total harmonic distortion (THD) of the ballast input current is preferred to be below 33.3% to prevent overheating of the neutral wire in a three phase power system. Further, many users of lighting systems require ballasts to have a ballast input current total harmonic distortion of less than 20%.
One approach to lowering the ballast input current total harmonic distortion and improving the ballast power factor has been to employ well known active power factor correction (APFC) circuits. This approach has certain tradeoffs including added ballast complexity, more components, greater cost, potentially lower reliability and, possibly, increased power consumption. Moreover, the ballast with APFC typically uses a relatively large bus capacitor with its attendant disadvantages as noted above.
Another approach to lowering ballast input current total harmonic distortion has been to employ a valley fill circuit between a rectifier and an inverter. One disadvantage of typical prior art valley fill circuits is that they can have greater bus ripple, which results in even higher lamp current crest factor, which can in turn shorten lamp life.
Prior art approaches to providing electronic ballasts having improved power factor and THD are discussed in T.-F. Wu, Y.-J. Wu, C.-H. Chang and Z. R. Liu, xe2x80x9cRipple-Free, Single-Stage Electronic Ballasts with Dither-Booster Power Factor Correctorxe2x80x9d, IEEE Industry Applications Society Annual Meeting, pp.2372-77, 1997; Y.-S. Youn, G. Chae, and G.-H. Cho, xe2x80x9cA Unity Power Factor Electronic Ballast for Fluorescent Lamp having Improved Valley Fill and Valley Boost Converterxe2x80x9d, IEEE PESC97 Record, pp. 53-59, 1997; and G. Chae, Y.-S. Youn, and G.-H. Cho, xe2x80x9cHigh Power Factor Correction Circuit using Valley Charge-Pumping for Low Cost Electronic Ballastsxe2x80x9d, IEEE 0-7803-4489-8/98, pp. 2003-8, 1998.
Prior art patents representative of attempts to provide electronic ballasts having improved power factor and total harmonic distortion include U.S. Pat. No. 5,387,847, xe2x80x9cPassive Power Factor Ballast Circuit for the Gas Discharge Lampsxe2x80x9d, issued Feb. 7, 1995 to Wood; U.S. Pat. No. 5,399,944, xe2x80x9cBallast Circuit for Driving Gas Dischargexe2x80x9d, issued Mar. 21, 1995 to Konopka et al.; U.S. Pat. No. 5,517,086, xe2x80x9cModified Valley Fill High Power Factor Correction Ballastxe2x80x9d, issued May 14, 1996 to El-Hamamsy et al.; and U.S. Pat. No. 5,994,847, xe2x80x9cElectronic Ballast with Lamp Current Valley-fill Power Factor Correctionxe2x80x9d, issued Nov. 30, 1999.
Another reference is xe2x80x9cFluorescent Ballast Design Using Passive P.F.C. and Crest Factor Controlxe2x80x9d by Peter M. Wood, 1998. This reference shows a ballast of the type employing a line frequency filter having a substantial impedance at the line frequency and about the first 30 harmonics of the line frequency.
In accordance with a first feature of the invention, a novel electronic ballast for driving a gas discharge lamp includes a rectifying circuit to convert an AC line input voltage to a rectified voltage, a valley fill circuit including an energy storage device which is charged through a switched impedance, the energy in this device being used to fill the valleys between successive rectified voltage peaks to produce a valley filled voltage, and an inverter circuit having series connected controllably conductive devices to convert the valley filled voltage to a high frequency AC voltage. The energy storage device can be a capacitor or an inductor or any other energy storage component or combination of components. Charging the energy storage device refers to increasing the energy stored in the energy storage device. A controllably conductive device is a device whose conduction can be controlled by an external signal. These controllably conductive devices include devices such as metal oxide semi-conductor field effect transistors (MOSFETs), insulated gate bi-polar transistors (IGBTs), bi-polar junction transistors (BJTs), Triacs, SCRs, relays, switches, vacuum tubes and other switching devices. The high frequency AC voltage is applied to a resonant tank circuit for driving a current through a gas discharge lamp, and a control circuit is provided for controlling the conduction of the controllably conductive devices in a novel way to deliver a desired lamp current to the gas discharge lamp and a reduced total harmonic distortion of the ballast input current. The electronic ballast of the invention described can drive more than one gas discharge lamp.
In a preferred embodiment of the ballast, the energy storage device of the valley fill circuit includes a capacitor, commonly referred to as the valley fill capacitor, that stores energy during a first charging portion of each half-cycle of the AC line voltage, and delivers energy to the inverter circuit which in turn drives lamp current through a gas discharge lamp during a second discharge portion of each half-cycle of the AC line voltage. The switched impedance of the valley fill circuit includes a resistor in series with a controllably conductive device, through which the valley fill capacitor is charged.
In an alternative embodiment, the energy storage device of the valley fill circuit includes a valley fill capacitor, and the switched impedance includes an inductor in series with a controllably conductive device, connected together in a buck converter circuit configuration. The valley fill capacitor stores energy during a first charging portion of each half-cycle of the AC line voltage, and delivers energy to the inverter circuit during a second discharge portion of each half-cycle of the AC line voltage. The buck circuit inductor stores energy in response to conduction of the controllably conductive device during the charging period of the valley fill capacitor, and transfers the stored energy to the valley fill capacitor in response to non-conduction of the controllably conductive device during the charging period of the valley fill capacitor.
In an alternative embodiment, the buck circuit inductor is provided with a tap connected to the bus voltage through a commutation diode to provide different charge and discharge times for the valley fill capacitor.
In accordance with a second feature of the invention, a novel electronic ballast for driving a gas discharge lamp includes a rectifying circuit to convert an AC line input voltage to a full wave rectified voltage, a valley fill circuit to fill the valleys between successive rectified voltage peaks to produce a valley filled voltage, an inverter circuit having series-connected switching devices (controllably conductive devices) to convert the valley filled voltage to a high-frequency AC voltage, a resonant tank for coupling the high-frequency AC voltage to a gas discharge lamp, a control circuit for controlling the conduction of the controllably conductive devices to deliver a desired current to the gas discharge lamp, and means for drawing input current near the zero crossing of the AC line input voltage so that the ballast input current total harmonic distortion is reduced.
In a preferred embodiment of the ballast, the means for drawing current near the zero crossing is a cat ear circuit. Preferably the cat ear circuit is a cat ear power supply that may also supply the power necessary to operate the control circuit or other housekeeping and auxiliary circuits. The cat ear circuit draws current from the AC line around the zero crossing of the AC line voltage at either the leading edge of each half-cycle, or the trailing edge of each half-cycle, or both. The cat ear circuit derives its name from the characteristic shape of its input current waveform. This current xe2x80x9cfills inxe2x80x9d or supplements the current waveform drawn by the ballast from the AC line around the zero voltage crossings. The cat ear circuit may be provided with circuitry that xe2x80x9ccuts inxe2x80x9d and xe2x80x9ccuts outxe2x80x9d the cat ear circuit in response to fixed input voltage levels. Alternatively, the cat ear circuit may be provided with circuitry to monitor the current drawn by the ballast back end and cause the cat ear circuit to draw input current only when the back end is not drawing significant current.